Figure  2-1  shows  one  hydrogen  and  one helium   atom.   Each   has   a   relatively   simple structure.  The  hydrogen  atom  has  only  one proton in the nucleus with one electron rotating about  it.  The  helium  atom  is  a  little  more complex.  It  has  a  nucleus  made  up  of  two protons  and  two  neutrons,  with  two  electrons rotating about the nucleus. Elements are classified numerically according to the complexity of their atoms.   The   atomic   number   of   an   atom   is determined   by   the   number   of   protons   in   its nucleus. In a neutral state, an atom contains an equal number  of  protons  and  electrons.  Therefore,  an atom of hydrogen, which contains one proton and one  electron,  has  an  atomic  number  of  1;  and helium, with two protons and two electrons, has an atomic number of 2. The complexity of atomic structure  increases  with  the  number  of  protons and electrons. MAGNETISM To understand properly the principles of how electrical  equipment  produces  work,  you  must understand magnetism, the effects of magnetism on  electrical  equipment,  and  the  relationship  of the different properties of electricity. Magnetism and electricity are so closely related that the study of either subject would be incomplete without at least  a  basic  knowledge  of  the  other. Much  of  today’s  electrical  and  electronic equipment  could  not  function  without  magnetism. Computers,  tape  recorders,  and  video  reproduction equipment  use  magnetic  tape.  High  fidelity speakers  use  magnets  to  convert  amplifier  outputs into audible sound. Electric motors use magnets to  convert  mechanical  motion  into  electrical energy.  Magnetism  is  generally  defined  as  that property of a material that enables it to attract pieces  of  iron.  Material  with  this  property  is known  as  MAGNETIC.  The  word   magnetic originated  from  the  ancient  Greeks,  who  found stones  possessing  this  characteristic.  Materials  that are  attracted  by  a  magnet,  such  as  iron,  steel, nickel,  and  cobalt,  have  the  ability  to  become magnetized.  Thus  they  are  magnetic  materials. Materials,   such   as   paper,   wood,   glass,   or tin,  which  are  not  attracted  by  magnets,  are considered nonmagnetic. Nonmagnetic materials Figure 2-2.—The effect of current. are not able to become magnetized. You will find additional information on the basic principles of magnetism  in  the  Navy  Electricity  and  Electronics Training  Series  (NEETS),  module  1,  NAVED- TRA  172-01-00-88,  chapter  1. ELECTRICITY Electricity  is  a  combination  of  a  force  called VOLTAGE   and   the   movement   of   invisible particles  known  as  CURRENT.  The  force  of voltage  can  be  compared  to  the  force  generated by a water pump, which moves water through a distribution system, generally an arrangement of pipes. Voltage is the force that causes current to flow  through  a  system  of  wires.  Current  is  the movement   of   invisible   particles   that   causes electrical  devices  to  operate.  We  cannot  see current, but we can determine its presence by the effects  it  produces.  Figure  2-2,  for  example,  shows the  effect  of  current.  It  shows  how  the  voltage force from a battery causes electrical current to flow through wires and an electrical motor. The current is invisible, but it produces the effect of making the motor run. Current flows through the wires much the same way as water flows through pipes. Current   consists   of   electrons,   which   are invisible  atomic  particles.  Voltage  is  the  force  that causes current, in the form of electrons, to move through  wires  and  electrical  devices.  However,  one important  difference  between  current  in  wires  and water  in  pipes  is  that  water  can  flow  out  of  a 2-3